The present invention relates to a centrifugal fan that collects airflow taken in from an air inlet formed at the center of one flat base wall of a scroll casing and discharges the airflow from an exhaust port formed on a circumferential side wall in a centrifugal direction. More particularly, the present invention relates to a mechanism to prevent backflow during fan operating.
Centrifugal fans, which use DC brushless motors especially, are widely used to cool electronic components of OA (office automatic) equipment such as a personal computer, a copying machine, a liquid crystal projector and a disk array because they can not only make the motors compact and light in weight but also control air quantity easily due to easy control of the motor.
A prior art of such a centrifugal fan will be described with reference to FIG. 6 through FIG. 8. FIG. 6 is a sectional view in a plane parallel to a rotating shaft showing a construction of a conventional centrifugal fan, FIG. 7 is a front view of the centrifugal fan shown in FIG. 6 viewed from an air inlet, and FIG. 8 is an enlarged sectional view of the upper half of the centrifugal fan shown in FIG. 6.
The illustrated centrifugal fan 1 has a scroll casing 10, an impeller 20 that is rotatably mounted in the casing 10, and a motor 30 that rotates the impeller 20. The casing 10 is provided with first and second flat base walls that are parallel to each other and a circumferential side wall that covers the circumferences of these base walls. The casing 10 is constructed by combining a first casing 10a that constitutes the first base wall and a second casing 10b that constitutes the second base wall and the circumferential side wall.
An air inlet 11 that opens in the axial direction is formed at the center portion of the first casing 10a and an exhaust port 12 (see FIG. 7) that opens in the circumferential direction is formed on one portion of the circumferential side wall. As shown in FIG. 6, the inner circumference of the air inlet 11 is inside to form a bell mouth 13.
A cylindrical bearing box 15 is formed on the second casing 10b. The bearing box 15 supports the rotating shaft 31 via bearings 14 in its inside. A stator 32 of the motor 30 is fixed to the outside of the bearing box 15.
The motor 30 is an outer-rotor type DC brushless motor that consists of a stator 32 having a stator core 32a and coils 32b wound in slots of the stator core 32a, and a rotor 33 having a cup-shaped hub 33a fixed on the tip of the rotating shaft 31 and a permanent magnet 33b attached to the inner circumferential surface of the hub 33a. 
The impeller 20 is fitted to the outer circumference of the hub 33a of the rotor 33. A great number of blades 21 are arranged around the outer circumference of the impeller 20. During fan operating, the impeller 20 rotates in a predetermined direction, which discharges the air taken in from the air inlet 11 to the periphery of the impeller 20 as regular airflow A by the centrifugal force as shown in FIG. 6. The air is collected by the inner circumferential surface of the casing 10, and is discharged from the exhaust port 12.
In the meantime, when the above-described centrifugal fan 1 operates with low air quantity, backflow B that flows in a space between the impeller 20 and the inner surface of the first casing 10a and is discharged from the air inlet 11 and recycling flow C that returns back to the impeller 20 are generated. The backflow B and the recycling flow C are generated because the regular flow A in radial direction tends to be concentrated to the side of the hub 33a during low air quantity operation. Particularly, the recycling flow C results from a velocity difference of airflow passing through a space between the impeller 20 and the inner surface of the first casing 10a. That is, the airflow at the side of the impeller 20 is slower than that at the side of the first casing 10a as shown in FIG. 8. Such backflow B and recycling flow C deteriorate the blowing performance of the centrifugal fan 1 and increase the noise.
Publications of Japanese unexamined patent applications No. Hei10-141291 and No. Hei10-054388 disclose techniques to prevent the deterioration of the blowing performance and the generation of the noise that are caused by the backflow B and the recycling flow C described above.
Namely, the publication of Japanese unexamined patent application No. Hei10-141291 discloses a centrifugal fan in which a screen-like guide plate is mounted on an outer portion of a casing at a periphery of an air inlet in order to return airflow discharged from the air inlet back to the air inlet. Further, an annular jutted portion is formed at a tip of an impeller so as to be inserted into a recess portion of a bell mouth formed having a U-shaped section.
However, since the guide plate is mounted on the outside of the casing in the construction of the publication, the size of the centrifugal fan in the axial direction (the axial size) becomes larger. Further, since the jutted portion is formed on the impeller, the inertial mass of the impeller becomes larger, which increases load on the motor.
Further, the publication of Japanese unexamined patent application No. Hei10-054388 discloses a centrifugal fan having labyrinth seal, which consists of a cylindrical shield plate mounted on an outer tip of an impeller and a cylindrical rib formed on a housing side, in order to prevent the backflow.
However, since the fan of the publication is constructed to reduce the backflow by seal effect, a high manufacturing accuracy is required to satisfy the seal effect, which increases a manufacturing cost. Further, since the shield plate is formed on the impeller, both the axial size of the impeller and the inertial mass of the impeller become larger, which increases the axial size of the centrifugal fan and load on the motor.